In recent years, the energy density of lithium-ion secondary batteries, nickel-hydrogen secondary batteries, and other types of batteries has been significantly increased. As a result, the size and weight of these batteries has been reduced, and the batteries can drive devices for a long time. The improvement performance of these batteries has greatly contributed to the popularization of cellular phones and other portable electronic devices.
Accompanying the improved battery performance is the improved performance of the peripheral circuits. For example, a battery device (known as battery pack) having an electronic circuit used for realizing various functions, such as control of the remaining capacity, in the battery main body can be loaded into a notebook computer, video camera, or another electronic device with replaceable batteries. In recent years, battery devices using a microcomputer to realize these functions has become the norm.
On the other hand, the characteristics of lithium-ion secondary batteries and other high-performance batteries tend to deteriorate when the cell voltage becomes excessively high due to overcharging or extremely low due to over-discharging or when an excessively large charging current flows through the cell, which creates problems. Therefore, a battery device usually includes a circuit that can protect the battery by shutting off the power supply path between the battery and the electronic device during abnormal charging/discharging. Japanese Kokai Patent Application No. 2005-160169 discloses a technology regarding a battery protection circuit loaded in a battery device.
Since the voltage and capacity of a cell as the smallest unit of a battery are determined by the type of cell, plural cells can be connected in series in order to realize the power supply voltage or power capacity required for the device. In general, there is a difference in voltage between the cells due to the differences in the initial voltage or characteristics during charging/discharging of the series connected cells. If charging/discharging is continued with the variation [difference in voltage] left as is, some cells may be over-charged or discharged. Consequently, the voltage of each cell is monitored to control charging/discharging of each cell of a lithium-ion secondary battery and other secondary batteries that require a high level of protection for over-charging or discharging.
In order to measure the voltage of series connected cells, a selector circuit that selects one cell from a plurality of cells and connects it to a voltage measurement system is required. In this selector circuit, MOS transistors are usually used as switches. A drive circuit turns on and off each switch by supplying a drive voltage to the gates of the MOS transistors. However, since the number of the series connected cells has been increased and the potential of the cell with respect to the reference potential of the drive circuit has been increased, a higher breakdown voltage will be required between the gate and source of the MOS transistor. In order to increase the gate-source breakdown voltage, it is necessary to adopt an appropriate design and manufacturing process for the MOS transistors, which may create problems in terms of manufacturing cost or element surface area.
The breakdown voltage problem can be avoided by forming the selector circuit in multiple stages. For example, two selector circuits are adopted in the first stage, and half of the upper part and lower part of the series connected cells are shared by said two selectors. The selector circuit in the next stage selects one of the two selection results of the first stage. In this way, the gate-source breakdown voltage required in the first stage is halved compared with that of the selector circuit constituted in only one stage. However, when said multi-stage constitution is adopted, the surface area will be increased since the number of the elements is increased. Also, the circuits scale will increase significantly as the number of the cells as the selection objects is increased.
On the other hand, since the number of the series connected cells is increased and the voltage is raised, the error becomes large when converting the potential of the cell selected by the selector circuit to the ground level of the measurement system.
Conventionally, a differential amplifier circuit that attenuates the common mode voltage by resistive voltage division is usually used to convert the cell voltage to ground level. However, since high accuracy is required for the resistance as the potential of the cell of the measurement object is increased, it is difficult to measure the voltage accurately.
A method that uses a switch and a capacitor to convert the voltage of each cell to ground level is also taken into consideration. However, since the parasitic capacitance of the transistor that constitutes the switch becomes the error of the measurement, the electrostatic capacitance of the capacitor must be much higher than said parasitic capacitance. As a result, the size of the capacitor will be increased.
A general object of the present invention is to provide a voltage converting circuit, which can effectively avoid the fact that the breakdown voltage required for the driving input of the switch element regarding selection of one power storage device from a plurality of power storage devices as the potential of the selected power storage device is increased.
Another object of the present invention is to provide a voltage converting circuit, that can accurately convert the voltage without increasing the size of the circuit elements.
A third object of the present invention is to provide a battery device which can restrain the increase in the circuit area and can uniformly control the voltages of the series connected power storage devices by equipping it with said voltage converting circuit.